Gas Mileage (MPG) Calculator

Using the gas mileage calculator

The calculator measures how far your vehicle travels on a known amount of fuel. The most reliable way to collect the numbers is the fill-up method:

1. Fill the tank until the pump clicks off and note the odometer reading (or reset the trip meter to zero).
2. Drive normally until the tank is at least half empty. A longer stretch smooths out the error from how full each tank really was.
3. Fill the tank again at the same pump if you can, stopping at the first click, and write down how much fuel it took.
4. Enter the odometer reading now as the current mileage, the earlier reading as the previous mileage, and the fuel added as gas added to tank.

If you used the trip meter, leave the previous mileage empty or enter 0. Choose US units for miles and gallons or Metric units for kilometers and liters; the price field changes from per gallon to per liter with the choice. The price is optional. When you provide one, the results include the cost of driving one mile (or one kilometer) and how far a single dollar takes you.

The US version reports three figures: miles per gallon, gallons per 1,000 miles (the line marked with an asterisk) and cost. The metric version reports liters per 100 km together with kilometers per liter, then cost per kilometer.

The MPG formula

Fuel economy is distance divided by the fuel used to cover it. The distance is the difference between the two odometer readings.

MPG = (current mileage − previous mileage) ÷ gallons added

gallons per 1,000 miles = gallons added ÷ (current − previous) × 1,000

cost per mile = price per gallon ÷ MPG

Worked example. The odometer read 41,250 miles at the last fill-up and reads 41,680 now, and the pump delivered 12.5 gallons at $3.60 per gallon.

• Distance driven: 41,680 − 41,250 = 430 miles.
• Fuel economy: 430 ÷ 12.5 = 34.4 MPG.
• Fuel efficiency: 12.5 ÷ 430 × 1,000 = 29.07 gallons per 1,000 miles.
• Cost per mile: $3.60 ÷ 34.4 = $0.10 per mile, which is about 9.56 miles for each dollar.

In metric units the same trip is 430 miles × 1.609344 = 692.02 km on 12.5 × 3.785411784 = 47.32 liters. That is 47.32 ÷ 692.02 × 100 = 6.84 liters per 100 km, or 14.62 km per liter.

The calculator uses US gallons (3.785 liters). An imperial (UK) gallon is about 4.546 liters, so an imperial MPG figure is roughly 20% higher than the US figure for the same car.

Converting between MPG, L/100 km and gallons per 1,000 miles

Because MPG measures distance per unit of fuel and L/100 km measures fuel per unit of distance, the two are reciprocals of each other, scaled by the unit conversion. For US gallons:

L/100 km = 235.215 ÷ MPG100 × 3.785411784 ÷ 1.609344 = 235.2146

gallons per 1,000 miles = 1,000 ÷ MPG

To convert a single value without a trip log, use the fuel consumption converter. To plan how much fuel a journey needs, try the fuel calculator or the fuel consumption calculator.

Fuel economy versus fuel efficiency: the MPG illusion

Fuel economy is how far you go per unit of fuel (MPG, km/l). Fuel efficiency, in the sense used by many regulators, is how much fuel you burn per unit of distance (gallons per 1,000 miles, L/100 km). The distinction matters because MPG is not proportional to fuel saved.

Compare two upgrades of equal size in MPG. Moving from 15 to 20 MPG changes consumption from 66.67 to 50 gallons per 1,000 miles, a saving of about 16.7 gallons. Moving from 35 to 40 MPG, also five MPG, changes it from 28.57 to 25 gallons, a saving of only 3.6 gallons. Gains at the thirsty end of the scale are worth far more than the same gain in MPG at the frugal end.

That is why the calculator also gives gallons per 1,000 miles: it is the number to use when you are comparing the running cost of two vehicles or estimating how much a more efficient replacement would save.

Why you should not average MPG figures

Averaging two MPG readings with a simple mean gives the wrong answer unless both legs used the same amount of fuel. Suppose you drive 100 miles at 20 MPG and another 100 miles at 40 MPG. The simple mean is 30 MPG, but the car used 100 ÷ 20 + 100 ÷ 40 = 7.5 gallons for 200 miles, which is 200 ÷ 7.5 = 26.67 MPG.

overall MPG = total miles ÷ total gallons

Always add up the distances and the fuel across trips and divide once. If you only have per-trip MPG values and equal distances, use the harmonic mean instead. The same trap catches anyone averaging speeds; see the average speed calculator.

How to improve your gas mileage

• Keep tires inflated to the pressure printed on the driver's door jamb. Under-inflated tires roll with more resistance.
• Drive smoothly. Hard acceleration and late braking waste energy, especially in town.
• Watch your highway speed. Aerodynamic drag grows with the square of speed, so economy usually falls noticeably above roughly 55 to 60 mph.
• Remove dead weight and roof racks. Extra cargo and drag from a rack or box both cost fuel.
• Avoid long idling. An idling engine burns fuel and covers zero distance.
• Keep up with maintenance. A clogged air filter, old spark plugs or the wrong oil grade can all cost efficiency.
• Combine short trips. A cold engine is at its least efficient in the first few miles.

Common measuring mistakes

• Using a single short tank. Pump cut-offs vary, so a few tanks averaged together are more reliable than one reading.
• Mixing units, such as kilometers from the odometer and gallons from the receipt. Use one consistent unit system.
• Topping off after the pump clicks. Squeezing extra fuel in distorts the fill measurement and is not good for the vehicle's vapor system.
• Relying on the dashboard average only. Onboard computers are convenient but can be off by a few percent, so check them against a real fill-up.